Device for reducing a metal ion from a salt melt

ABSTRACT

Between an anode and a cathode, a salt melt containing a metal ion is separated from the anode by a gap across which an electric arc can be formed. The metal ion is deposited on the anode and subsequently removed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage of International ApplicationNo. PCT/EP2014/062216, filed Jun. 12, 2014 and claims the benefitthereof. The International Application claims the benefit of GermanApplication No. 10 2013 211 922.4 filed Jun. 24, 2013, both applicationsare incorporated by reference herein in their entirety.

BACKGROUND

Described below is an apparatus for reducing a metal ion from a saltmelt.

Rare earth elements, which are also referred to as lanthanides inchemistry, are required in many electronic components and in theproduction of magnets. For example, the rare earth element neodymium isan important constituent of permanent magnets which are used in windgenerators. The work-up and separation of rare earth elements is inprinciple chemically complicated since the rare earth elements occur innature in very finely distributed and associated (especially with oneanother) form and in low concentrations. The rare earth elements arefrequently present in phosphate compounds, in particular in the crystalstructure of monazite or xenotime or as separate constituents inapatite, which are again finely distributed in deposits, which can alsocontain iron. A substep of this complicated process for obtaining rareearth elements in pure form is an electrolysis process in whichchlorides or fluorides of the rare earth element in molten form may beused as electrolyte. Application of a voltage between immersed graphiteanode and inert tungsten cathode results in the rare earth oxidesdissolved in the electrolyte being converted into metal and CO/CO₂.However, perfluorocarbons such as CF₄ or C₂F₆, which frequently have thegreenhouse potential of CO₂, are also formed at the carbon anode.Furthermore, highly toxic hydrofluoric acid can be formed in thepresence of water. All these undesirable products which are formed inthe electrolysis have to be got rid of again by complicated purificationand neutralization processes, which considerably increases the totalprocess costs. Similar problems occur in principle in the electrolysisof salt melts using graphite electrodes, for which reason application tothe preparation of rare earth elements can be considered to beillustrative.

SUMMARY

Described below is an apparatus which provides for the reduction ofmetal ions from metal-containing melts, in which there is a loweremission of damaging greenhouse gases compared to the prior art.

The apparatus for reducing a metal ion in a salt melt has an anode and acathode. The apparatus is wherein a gap for formation of an electric arcis present between the anode and the salt melt. The metal ion may be arare earth metal ion which is frequently prepared by electrolysis ofsalt melts. However, the apparatus is not restricted to the use of rareearth metal ions. Furthermore, the salt melt also contains oxygen ionswhich is due to the rare earth metal ion originally being present insolid form in the form of an oxide. An oxide is for the present purposesalso subsumed under the term salt.

Compared to a known electric arc melting pot, the apparatus describedhas the difference that the electric arc is present across a gap betweenthe anode and the surface of the salt melt. This in turn means, incontrast to the prior art in which graphite electrodes for the reductionof rare earth ions are dipped into the melt, that no carbon compoundswhich would form compounds with the anions, i.e., halide ions or oxygenions, are formed. Thus, no carbon halides which are damagingparticularly in terms of the greenhouse effect are formed. Furthermore,no hydrogen fluoride, i.e., no hydrofluoric acid, which is likewisehighly toxic is formed in the case of this apparatus.

The term rare earth elements refers, in particular, to the lanthanides,including, inter alia, lanthanum, cerium, praseodymium, neodymium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,ytterbium and lutetium, but yttrium and scandium are also counted asrare earth elements in this case because of their chemical similarities.Rare earths are in turn compounds of rare earth elements, in particularthe oxides thereof, but no rare earth phosphates are included here.

It has been found to be advantageous for the anode to be formed of achemically inert material having good conductivity, for example copper,which if necessary is cooled from the inside. This avoids any compoundbetween anions which are oxidized to the corresponding elements in theregion of the electric arc and the material of the anode. It has beenfound to be particularly advantageous for the salt melt to containoxygen ions, particularly instead of halide ions. The oxidation of theoxygen ions forms pure oxygen which is discharged as O₂ via the offgas.

In an advantageous embodiment, an electrolysis vessel which serves toaccommodate the salt melt is provided. This electrolysis vessel or thevessel wall thereof is in direct electrical contact with the cathode. Inprinciple, electrically conductive constituents of the electrolysisvessel can likewise serve as cathode. This means that in an electrolysisoperation, the positively charged cations, i.e., the metal ions, inparticular rare earth metal ions, are deposited on the vessel wall andas a result of their high specific gravity settle at the bottom of theelectrolysis vessel. This in turn leads to the elemental rare earthmetal constituents, whether in solid or liquid form, being in electricalcontact with the vessel wall and thus with the cathode and in turnacting as cathode. At the phase interface between the particles alreadyprecipitated as elemental metal and the salt melt, ever more metal atomsare deposited, so that a phase of pure metal is present in the lowerregion of the electrolysis vessel and can be separated off after theelectrolysis process.

A plasma may be present above the salt melt, i.e., in the region of ahollow space above the salt melt, in which the anode is also arranged.For the present purposes, a plasma is an ionized gas, for example anionized noble gas. As plasma gas, a mixture of argon and nitrogen may beused. This gas is also referred to as inert gas since it undergoes achemical reaction neither with the salt melt nor with the material ofthe anode. In a further advantageous embodiment, the salt melt includesnot only the oxide of the metal to be reduced, i.e., generally the rareearth metal, but also further oxides. These are oxides of metals whichare more stable in respect of the electrolysis than the rare earth metaloxide and at the same time reduce the melting point of the salt melt. Inprinciple, other salts can also be employed for reducing the meltingpoint as long as these are sufficiently stable, in particular in respectof their anions, for no damaging halides to be formed at the anode.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages will become more apparent andmore readily appreciated from the following description of the exemplaryembodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a sequence chart using schematic drawings of a process forextraction of rare earth metals from an ore; and

FIG. 2 is a schematic block diagram of the electrolysis of a salt melt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments,examples of which are illustrated in the accompanying drawings, whereinlike reference numerals refer to like elements throughout.

Firstly, the process for extraction of rare earth metals, as is, forexample, customary for the mineral monazite, is shown schematically inFIG. 1, without making any claims as to completeness. The mineralmonazite is a phosphate in which the metal ions frequently occur in theform of rare earth metals, in particular cerium, neodymium, lanthanum orpraseodymium. Here, there is not a homogeneous composition in respect ofrare earth metals within a particle, but instead the lattice sites ofthe cations in the crystal structure are occupied by various rare earthmetals in different concentrations.

The starting raw materials containing the monazite mineral are firstlymilled very finely and treated in a flotation plant 2 in such a way thatthe monazite is separated as well as possible from the other mineralconstituents. The monazite is dried and, according to the related art,admixed with sulfuric acid and then treated in a furnace, for example arotary tube furnace 4. Here, the phosphates are converted into sulfates.This process in the rotary tube furnace takes place at temperatures upto 650° C. The conversion of phosphate into sulfate is advantageoussince the rare earth sulfates are significantly more readily soluble inwater than the phosphates of the rare earth metals.

The sulfuric acid-containing solution of rare earth sulfates is, aftertreatment in the rotary tube furnace 4 and a subsequent leaching,neutralized in a neutralization apparatus 6, i.e., the pH is increasedby addition of a basic substance, resulting in undesirable substancesbeing precipitated and separated off so that an aqueous rare earthsulfate solution is present in the remaining liquid.

This resulting solution of a rare earth compound (sulfate, nitrate,chloride or the like) is usually subjected to a liquid/liquidextraction, i.e., a separation, in mixer-settler apparatuses 8. Here,the solution is treated by mixing with an extractant dissolved inorganic solvents such as kerosene, including possible further additives,in such a way that the rare earth cations which in the case of the samecharge have slightly different ion diameters accumulate at differentconcentrations either in the aqueous part of the solution or in theorganic part of the solution. The organic phase and the aqueous phase ofthe mixture are here alternately mixed and separated again in amultistage separation process, so that particular rare earth ionsbecome, depending on the extractant in the organic phase, ever moreconcentrated until these ions are present in sufficient purity in onephase. Up to 200 separation operations per element can be necessaryhere.

The rare earth metals which have been separated in this way aresubsequently precipitated by addition of a carbonate or oxalate in aprocess in a precipitation apparatus 10, so that the respective rareearth carbonate or oxalate accumulates at the bottom of theprecipitation apparatus 10. This is in turn calcined in a calcinationapparatus, for example in a tunnel kiln 12, through which a stream ofhot air is passed. After this process, a discrete rare earth oxide isthus present.

This discrete rare earth oxide is continuously added to a moltenelectrolyte in the electrolysis plant 16. The electrolyte is mainlyformed of the corresponding rare earth fluoride. The oxide compounddissociates into rare earth cations and oxygen anions in thiselectrolyte. The rare earth cations are reduced to elemental metal atthe cathode and are collected in a collection vessel underneath thecathode. The oxygen ions react with the carbon of the anode to formCO/CO2, but fluorine ions also form compounds with the carbon of theanode and leave the electrolysis bath together in gaseous form.

The rare earth oxide can optionally be converted into a lower-meltingsalt, e.g. an iodide, a chloride or fluoride, before introduction intothe electrolysis process and then be introduced in molten form into anelectrolysis process, with elemental rare earth metal depositing at acathode of the electrolysis apparatus.

The metal 20 obtained in liquid form is pumped out from the collectionvessel underneath the cathode and cast to produce ingots.

FIG. 2 illustrates an advantageous embodiment of an electrolysisapparatus. This is a schematic depiction of an electrolysis apparatus.The apparatus has an anode 26 and a cathode 28. A salt melt 24 isaccommodated in an electrolysis vessel 34. This salt melt 24 can beheated either by a resistance heating element (not shown here) or by anelectric arc 32 which generates a plasma 33. A combination of aplurality of heating methods is also possible. A gap 30 is providedbetween the anode 26 and a surface 42 of the salt melt 24 and anelectric arc 32 is present in this gap when a voltage is applied. Thiselectric arc 32 leads to inert gas, in particular a mixture of argon andnitrogen, which is introduced via an inert gas feed line 36 beingionized and being present in the form of a plasma 33 above the surface42. In a plasma space 44, in which the plasma 43 is present and which islargely sealed off from an atmosphere, a positive charge prevails. Thenegative charges of the salt melt 24, in particular oxygen ions, migrateto the surface 42 of the salt melt, also referred to as electrolyte, andare oxidized there to atomic oxygen at the boundary between the saltmelt, i.e., the electrolyte, and the plasma. This means that theelectrolyte should be conductive for rare earth ions, oxygen ions andalso electrons. The atomic oxygen forms O2 molecules outside the plasmaspace 44 and leaves the plasma space through the offgas outlet 38.

The anode is a material which is self evidently firstly electricallyconductive but on the other hand is inert to all reactants in theelectrolysis system. For this purpose, the anode has to have internalwater cooling so that it does not melt at the high plasma temperatures.It is possible to use, for example, copper as material here. However,the anode does not consist of carbon since carbon together with theoxidized elements, in particular with the oxygen but also with certainhalides if they are present in the salt melt, tends to form gases whichcause great damage to the atmosphere, in particular are stronggreenhouse gases.

In contrast to the anode arranged above the salt melt, the cathode iselectrically conductively connected to a vessel wall 40 of theelectrolysis vessel underneath the salt melt. In principle, the vesselwall 40 can also be formed of an electrically conductive material andthus directly form the cathode 28. In this case, it would beadvantageous for upper regions of the vessel wall or of the electrolysisvessel 34 to be electrically insulated from lower regions. As analternative, it is also possible to make the electrolysis vessel of arefractory material which in its lower region has a cutout into which ametallic or other conductive cathode 28 is inserted. On application ofan appropriate voltage, elemental metal which has formerly been presentin the form of metal ions in the salt melt 24 is deposited at theelectrically conductive cathode 28. The surface of the cathode 28 isthus covered very promptly by elemental metal, but this is likewiseelectrically conductive and thus builds up a fresh electricallyconductive surface at which further ions can again be reduced. Theelectrolysis is stopped when there is no longer any voltage or when thesalt melt 24 is present in chemical equilibrium and no furtherelectrolysis takes place. Depending on the temperature in theelectrolysis vessel, i.e., depending on the melting point of theelectrolyte 24 or salt melt 24 used, and depending on the melting pointof the metal being deposited, the latter can be present either in solidform or in liquid form at the cathode 28 in the lower region of theelectrolysis vessel 34. Accordingly, the deposited metal, i.e., the rareearth metal 20, can be drained off when it is present in liquid form orcan be taken out in pure, solid form after solidification of the saltmelt 24.

A substantial advantage of the apparatus is firstly that there is aspacing between the anode 26 and the electrolyte 24 or the salt melt 24,i.e., the materials of the electrode do not come into direct contactwith the salt melt 24 but are instead connected to one another in energyterms only indirectly via the electric are 32. A further important pointis that, compared to known electric arc processes, the polarity isreversed so that the anode is positioned above the salt melt and theelectric arc 32 prevails between the anode and the salt melt. This inturn leads to the now elemental, oxidized anions, which are generallypresent in gaseous form, rising upward and being able to escape from theapparatus via the plasma space 44 and the offgas outlet 38. Furthermore,it is possible as a result of this arrangement for the elemental metalto be isolated as material value to settle on the bottom of theapparatus at the cathode 28. Thus, a high measure of purity of thedeposited metal 20 can also be achieved here.

A further advantage is to select the material of the salt melt 24 insuch a way that very few halides and a large amount of oxygen ions arepresent, so that no damaging halogen compounds or elemental halogensoccur in the oxidation of the anions. However, since the halogencompounds are not compounds with carbon, salts can also be present inthe form of halides in the salt melt 24 when this serves to lower themelting point of the salt melt 24. Overall, production of CO2 isprevented and any after-treatment of the offgas becomes significantlysimpler and less costly. This serves to make the ecologicallyproblematical process for extraction of rare earth metals or othermetals cheaper and more ecologically friendly.

A description has been provided with particular reference to preferredembodiments thereof and examples, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the claims which may include the phrase “at least one of A, B and C”as an alternative expression that means one or more of A, B and C may beused, contrary to the holding in Superguide v. DIRECTV, 358 F3d 870, 69USPQ2d 1865 (Fed. Cir. 2004).

1-10. (canceled)
 11. An apparatus for reducing a metal ion in a saltmelt, comprising: a cathode; and an anode disposed above the salt meltwith a gap for formation of an electric arc therebetween.
 12. Theapparatus as claimed in claim 11, wherein the salt melt comprises oxygenions.
 13. The apparatus as claimed in claim 11, wherein the salt meltcomprises a rare earth metal ion.
 14. The apparatus as claimed in claim11, wherein the anode is inert toward materials in the salt melt. 15.The apparatus as claimed in claim 11, further comprising an electrolysisvessel accommodating the salt melt; and wherein the cathode iselectrically connected to a wall of the electrolysis vessel.
 16. Theapparatus as claimed in claim 15, wherein the cathode is arranged at abottom of the electrolysis vessel.
 17. The apparatus as claimed in claim11, wherein a plasma prevails above the salt melt.
 18. The apparatus asclaimed in claim 17, wherein an inert gas which forms the plasma ispresent above the salt melt.
 19. The apparatus as claimed in claim 11,further comprising an inert gas feed line and an offgas outlet, andwherein the salt melt has a surface separated from air surrounding theapparatus.
 20. The apparatus as claimed in claim 11, wherein the saltmelt comprises an oxide of the metal ion to be reduced and additionaloxides.